Lubricated contacts in technical systems frequently suffer damage due to material fatigue. This can lead to the failure of the entire technical system.
One form of material fatigue is so-called “white etching cracks” (shortened to WEC below). Here, in the contact area of machine elements, for example, when machine elements roll or slide over each other, cracks are created and spread on and under the material surface. In roller bearings, for example, WECs in the rolling contact can occur on or near the raceway surface and can grow under rolling loading as partially branching fatigue cracks into the depth of the material. In practice, the appearance of such WECs leads to the result that large roller bearings that are installed, for example, in wind turbines, in individual cases fail long before their expected statistical service life. The spontaneous and unpredictable occurrence of WECs is associated with high failure and repair costs. WECs are also called “white structured flaking” (WSF).
The cause of the WEC damage mechanism is not conclusively known according to the current state of knowledge. Material solutions and processing methods that largely prevent WECs are known. These solutions are expensive and therefore rarely used in comparison with standard materials and methods.
In DE102007055575A1 it is proposed to combat the occurrence of WECs by generating internal compressive stresses in the area of the raceways. Such internal compressive stresses can be generated by a material-ablating processing method.
In EP2123779A1, for increasing the fatigue strength and for reducing the risk of the occurrence of WECs it is proposed to produce raceways and/or roller elements from a hardened steel with a carbon percentage between 0.4 and 0.8 weight percent.
In EP2573195A1, for increasing the robustness relative to WECs, it is proposed to produce the surface of a roller element or a raceway from a modified material, wherein the roller bearing is exposed for a certain period of time to an increased temperature, while the bearing surface is in contact with a chemical additive.
It is further known that WEC-conditional fatigue damage of lubricated machine elements is associated with the lubricant being used. Therefore, for example, in WO2012/022501A1, WO2007/010845A1, EP2022840A2, US2009/0069204A1, WO2005/035702A1, and Ye, et al., Chem. Commun. 2001, 2244-2245, lubricant solutions with ionic fluid are proposed.
The chemical composition of the lubricant, however, changes during the course of the operation of the lubricated machine element. Therefore, lubricant samples that are typical in practice are taken and sent to a laboratory for study. Based on the study results, it is indicated whether the examined chemical composition of the lubricant is already critical. In the presence of a critical lubricant composition, one possible action is to flush the bearing and replace the lubricant.
The process of laboratory analysis is lengthy, wherein, under some circumstances, a quick-enough response is not possible and fatigue damage can nevertheless occur within the analysis interval. Furthermore, it is not possible to determine a WEC failure tendency, that is, a likelihood for the appearance of WECs. Frequent and regular maintenance, for example, in the form of a flushing of lubricated machine elements, however, under some circumstances also leads to the operational failure of the technical system and is thus expensive.